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<p class="MsoNormal" style="margin-left:36.0pt">There is also the small matter, in this example, of distinguishing which `_' is which. The description works, but you have to think about it. I don't have an immediate and simple solution to this. Perhaps the
addition of unique labels (e.g. _$1 _$2). But this is not a major problem. It can even wait until some future development/expansion on TypeHoles.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">I have a proposal. Someone has already suggested on hackage.haskell.org/trac/ghc/ticket/5910 that an un-bound variable behaves like a hole. Thus, if you say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f x = y<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">GHC says “Error: y is not in scope”. But (idea) with -XTypeHoles<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">f x = y<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">might generate
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">(renamer)
<b>Warning</b>: y is not in scope<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">(type)
<b>Error</b>: Hole “y” has type....<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">So that’s like a named hole, in effect.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">If you say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f x = 4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">GHC warns about the unused binding for x. But if you say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f _x = 4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">the unused-binding warning is suppressed. So (idea) if you say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f x = _y<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">maybe we can suppress warning (1). And, voila, named holes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Moreover if you add –fdefer-type-errors you can keep going and run the program.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Any comments? This is pretty easy to do.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">(I’m unhappy that –XTypeHoles is a language pragma while –fdefer-type-errors is a compiler flag. Maybe we should have –XDeferTypeErrors?)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> sean.leather@gmail.com [mailto:sean.leather@gmail.com]
<b>On Behalf Of </b>Sean Leather<br>
<b>Sent:</b> 03 October 2012 16:45<br>
<b>To:</b> Simon Peyton-Jones<br>
<b>Cc:</b> GHC Users List; Thijs Alkemade<br>
<b>Subject:</b> Comments on current TypeHoles implementation<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi Simon,<o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Thanks for all your work in getting TypeHoles into HEAD. We really appreciate it.<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I was playing around with HEAD today and wanted to share a few observations.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">(1) One of the ideas we had was that a hole `_' would be like `undefined' but with information about the type and bindings. But in the current version, there doesn't appear to be that connection. This mainly applies to ambiguous type variables.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Consider:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">> f = show _<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">The hole has type a0.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">But with<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">> f = show undefined<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">there is a type error because a0 is ambiguous.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">We were thinking that it would be better to report the ambiguous type variable first, rather than the hole. In that case, tou can use -fdefer-type-errors to defer the error. Currently, you don't have that option. I can see the argument
either way, however, and I'm not sure which is better.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">(2) There is a strange case where an error is not reported for a missing type class instance, even though there is no (apparent) relation between the missing instance and the hole. (This also relates to the connection to `undefined', but
less directly.)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">We have the following declaration:<o:p></o:p></p>
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<div>
<div>
<p class="MsoNormal">> data T = T Int {- no Show instance -}<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">With a hole in the field<o:p></o:p></p>
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<div>
<p class="MsoNormal">> g = show (T _)<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal">we get a message that the hole has type Int.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">With<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">> g = show (T undefined)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">we get an error for the missing instance of `Show T'.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">(3) In GHCi, I see that the type of the hole now defaults. This is not necessarily bad, though it's maybe not as useful as it could be.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">ghci> :t show _<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">reports that the hole has type ().<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">(4) In GHCi, sometimes a hole throws an exception, and sometimes it does not.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">ghci> show _<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">throws an exception with the hole warning message<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">ghci> show (T _)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">and<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">ghci> _ + 42<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">cause GHCi to panic.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">(5) There are some places where unnecessary parentheses are used when pretty-printing the code:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<div>
<p class="MsoNormal">ghci> :t _ _<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><interactive>:1:1: Warning:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> Found hole `_' with type t0 -> t<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> Where: `t0' is a free type variable<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> `t' is a rigid type variable bound by<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> the inferred type of it :: t at Top level<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> In the expression: _<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> In the expression: _ (_)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><interactive>:1:3: Warning:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> Found hole `_' with type t0<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> Where: `t0' is a free type variable<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> In the first argument of `_', namely `_'<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> In the expression: _ (_)<o:p></o:p></p>
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<div>
<p class="MsoNormal">_ _ :: t<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">The argument `_' does not need to be printed as `(_)'.<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">There is also the small matter, in this example, of distinguishing which `_' is which. The description works, but you have to think about it. I don't have an immediate and simple solution to this. Perhaps the addition of unique labels (e.g.
_$1 _$2). But this is not a major problem. It can even wait until some future development/expansion on TypeHoles.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Regards,<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Sean<o:p></o:p></p>
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